Mattis replied: "Our ballistic-missile-defense forces at sea and in Alaska and California ... the various radars would be feeding in, and they would do what they're designed to do as we make every effort to take them out."

"The response — if that's what you're referring to — would, of course, depend on the president," Mattis said. He explained that the president would see a "wide array" of options that included cooperation with US allies.

"Defenses will go," Mattis said. "The president will be woken up or whatever, but our commands are — we rehearse this routinely."

Secretary of State Rex Tillerson added: "Some judgment would be made over whether a necessary and proportionate response is required."

But neither Tillerson nor Mattis would categorically rule out a nuclear first strike on North Korea. Both made statements to the effect that if the US knew a North Korean nuclear attack on the US was imminent, President Donald Trump reserved the right to preempt it with a launch.

"The fact is that no president, Republican or Democrat, has ever forsworn the first-strike capability," Tillerson said. "That has served us for 70 years."